This invention relates to a radiation image storage panel by use of a stimulable phosphor and a process for making the same, more particularly to a radiation image storage panel which can give a radiation image of high sharpness and a process for making the same.
A radiation image such as X-ray image is frequently used in diagnosis of diseases, etc. For obtaining such an X-ray image, the so-called radiation photograph is utilized, which is obtained by irradiating X-ray transmitted through a subject on a phosphor layer (fluorescent screen) to thereby generating a visible light, which visible light is then irradiated on a film employing a silver salt similarly as in conventional photographing, followed by development. However, in recent years, methods for taking out images directly from the phosphor layer without use of a film coated with a silver salt have been devised.
As such a method, there is the method for imaging in which a radiation transmitted through a subject is absorbed onto a phosphor, then the phosphor is excited with, for example, light or heat energy to thereby permit the radiation energy accumulated in this phosphor by the above absorption to be radiated as fluorescence, which fluorescence is detected. Specifically, for example, U.S. Pat. No. 3,859,527 and Japanese Provisional Patent Publication No. 12144/1980 discloses a radiation image storage method employing visible light or IR-ray as the stimulating excitation light. This method employs a radiation image storage panel having a stimulable phosphor layer formed on a support. By irradiating the stimulable phosphor layer in this radiation image storage panel with the radiation transmitted through a subject, the radiation image corresponding to the radiation transmission degrees at respective portions of the subject to form a latent image and thereafter the stimulable phosphor layer is scanned with a stimulating excitation light to thereby radiating the radiation energy accumulated for the respective portions, which is converted to light to obtain an image according to the optical signals depending on the intensity of the light. The final image may be reproduced as a hard copy or reproduced on CRT.
Now, the radiation image storage panel having a stimulable phosphor layer to be used for the radiation image storage method is demanded to be high in radiation absorption and light conversion (hereinafter referred to as "radiation sensitivity" as inclusive of both) as a matter of course, good in graininess of the image and yet high in sharpness.
Whereas, radiation image storage panels having stimulable phosphor layers are generally made by applying and drying a dispersion containing a stimulable phosphor in grains of about 1 to 30 .mu.m grain sizes and an organic binder, and therefore low in filling density of the stimulable phosphor (filling percentage 50%). Accordingly, in order to make the radiation sensitivity sufficiently high, it is necessary to make the layer thickness of the stimulable phosphor layer thick as shown in FIG. 12(a), line (b).
As apparently seen from the same Figure, the amount of the stimulable phosphor attached is 50 mg/cm.sup.2 when the layer thickness of the stimulable phosphor layer is 200 .mu.m, and the radiation sensitivity is increased linearly until saturated at 450 .mu.m or more. In this regard, the radiation sensitivity is saturated, because stimulated emission within the stimulable phosphor layer will not come out due to scattering of the stimulable phosphor layer between the stimulable phosphor grains, if the stimulable phosphor layer becomes too thick.
On the other hand, sharpness of the image in the above radiation image storage method tends to become higher as the layer thickness of the stimulable phosphor layer in the radiatio-n image storage panel is thinner, as shown in FIG. 13(b), line (b), and therefore it is necessary to make the stimulable phosphor layer thinner for improvement of sharpness.
Also, since the graininess of the image in the above radiation image storage method is determined by the regional fluctuation in radiation quantum number (quantum mottle) or structural disturbance (structural mottle) of the stimulable phosphor layer of the radiation image storage panel, if the layer thickness of the stimulable phosphor layer becomes thin, the radiation quantum number absorbed by the stimulable phosphor layer may be reduced to increase quantum mottle or the structural disturbance may be actualized to increase structural mottle, thereby causing lowering in image quality. Hence, for improvement of graininess of the image, the layer thickness of the stimulable phosphor is required to be thick.
Thus, as described above, the radiation image storage panel of the prior art exhibits tendency for sensitivity to radiation and graininess of image entirely opposite to that for sharpness of the image in relation to the layer thickness of the layer thickness of the phosphor layer, and therefore the above radiation image storage panel has been made at the sacrifice of sensitivity to radiation, graininess and sharpness to some extent.
Whereas, sharpness of the image in the radiation photographic method of the prior art is determined by expansion of the momentary emission (emission on irradiation of radiation) of the phosphor in the fluorescent screen, as is well known in the art. In contrast, sharpness of the image in the radiation image storage method utilizing a stimulable phosphor as described above is not determined by the expansion of the stimulated emission of the stimulable phosphor in the radiation image storage panel, namely by the expansion of the emission of the phosphor as in the radiation photographic method, but determined depending on the expansion of the stimulating excitation light within said panel. For, in this radiation image storage method, since the radiation image information accumulated in the radiation image storage panel is taken out as arranged in a time series, the stimulated emission by the stimulating excitation light irradiated at a certain time (ti) is desirably all collected and recorded as the output from a certain picture element (xi,yi) on said panel on which the stimulating excitation light is irradiated at that time, and therefore, if the stimulating excitation light is expanded by scattering, etc. within said panel and also excites the stimulable phosphor existing outside of the irradiated picture element (xi,yi), then the output from wider region than the picture element is recorded as the output from the above picture element of (xi,yi). Thus, provided that the stimulated emission by the stimulating excitation light irradiated at a certain time (ti) is only the emission from the picture element (xi,yi) on said panel on which the stimulating excitation light is truly irradiated at that time (ti), there is no influence on sharpness of the image obtained even if the emission may have any expansion.
In the state of the art as described above, some methods for improving sharpness of the radiation image have been invented. For example, there are the method in which white powder is mixed into the stimulable phosphor layer in the radiation image storage panel as disclosed in Japanese Provisional Patent Publication No. 146447/1980; the method in which the radiation image storage panel is colored so that the average reflectance in the stimulating excitation wavelength region of the stimulable phosphor is made smaller than the average reflectance in the stimulated emission wavelength region of the above stimulable phosphor, etc. However, these methods will necessarily lower markedly sensitivity, if sharpness is improved, and therefore cannot be said to be preferable methods.
On the other hand, as contrasted to these methods, the present applicant has already proposed in Japanese Patent Application No. 196365/1984 a radiation image storage panel in which the stimulable phosphor layer contains no binder as a novel radiation image storage panel which has improved the drawbacks of the prior art in the radiation image storage panel employing a stimulable phosphor as described above. According to this proposal, since the stimulable phosphor layer in the radiation storage panel contains no binder, filling percentage of the stimulable phosphor can be improved simultaneously with improvement of inclination of the stimulable excitation light and the stimulable emission, whereby sensitivity of the above radiation image storage panel to radiation and graininess of the image can be improved simultaneously with improvement of sharpness of the image.
However, in the above radiation image storage method, demand for image quality excellent in sharpness without damaging sensitivity and graininess is becoming more rigorous.